It has long been known that exhaust gases produced by combusting hydrocarbon fuels can contribute to atmospheric pollution. Exhaust gases typically contain pollutants such as nitric oxide (NO) and nitrogen dioxide (NO.sub.2), which are frequently grouped together as NO.sub.x, unburned hydrocarbons (UHC), carbon monoxide (CO), and particulates, primarily carbon soot. Nitrogen oxides are of particular concern because of their role in forming ground level smog and acid rain and in depleting stratospheric ozone. NO.sub.x may be formed by several mechanisms. First, the high temperature reaction of atmospheric oxygen with atmospheric nitrogen, particularly at adiabatic flame temperatures above about 2800.degree. F., forms "thermal NO.sub.x " through the Zeldovich mechanism. Second, the reaction of atmospheric nitrogen with hydrocarbon fuel fragments (CH.sub.i), particularly under fuel-rich conditions, forms "prompt NO.sub.x ". Finally, the reaction of nitrogen released from a nitrogen-containing fuel with atmospheric oxygen, particularly under fuel-lean conditions, forms "fuel-bound NO.sub.x ". In typical combustors, atmospheric oxygen and nitrogen are readily available in the combustion air which is mixed with the fuel.
Various combustor strategies can be employed to decrease the formation of thermal and fuel-bound NO.sub.x. For example, the combustor may be configured to operate uniformly fuel-lean, that is, at equivalence ratios of less than 1.0. The equivalence ratio (.PHI.) is the ratio of the actual fuel/air ratio to the fuel/air ratio required for stoichiometric combustion. An equivalence ratio of greater than 1.0 indicates fuel-rich conditions, while an equivalence ratio of less than 1.0 indicates fuel-lean conditions. At low equivalence ratios, the adiabatic flame temperatures may be sufficiently low that thermal NO.sub.x does not form in appreciable quantities despite the presence of nitrogen and oxygen. This approach, however, can be limited by combustion stability considerations. Moreover, lean combustion does not reduce the formation of fuel-bound NO.sub.x.
An alternative low NO.sub.x combustor configuration uses geometrically or aerodynamically separated rich and lean combustion stages. The fuel is first mixed with air and combusted under fuel-rich conditions. The combustion products from the fuel-rich combustion are then rapidly mixed with additional air and combusted under fuel-lean conditions. This operation is sometimes referred to as rich burn/quick quench/lean burn combustion. The staged rich/lean combustor provides the capability to control both fuel-bound and thermal NO.sub.x emissions without the combustor stability limitations which can accompany uniformly lean combustion. Nitrogen species contained in the fuel are released in the fuel-rich combustion stage and, because of the low oxygen concentration, do not react to form fuel-bound NO.sub.x. The fuel-lean combustion stage can be operated at low adiabatic flame temperatures to avoid forming appreciable amounts of thermal NO.sub.x. Combustion stability limitations are avoided because the combustion products from the rich stage are very hot, promoting rapid reaction rates in the lean stage and, therefore, stable combustion.
Studies evaluating the potential of staged rich/lean combustion to control NO.sub.x emissions have concluded that NO.sub.x emissions can be minimized by operating the rich combustion stage at a global equivalence ratio (.PHI..sub.R) of about 1.5 to about 1.8. The precise value of .phi..sub.R which yields minimum NO.sub.x may be influenced by combustor residence time, but depends only slightly on fuel type. Optimization of the rich combustion stage may be limited by soot formation, which increases as both the global and local equivalence ratios are increased. Soot is undesirable because it can greatly increase heat transfer to the rich combustor liner and can persist as visible smoke emissions. Experimentally observed trends indicate that over the range 1.0&lt;.phi..sub.R &lt;2.0, soot production increases continuously, while minimum NO.sub.x production occurs at a finite .phi..sub.R. Therefore, there might be a trade-off between increasing .phi..sub.R to minimize NO.sub.x emissions and decreasing .phi..sub.R to limit soot formation. However, equilibrium thermochemical calculations predict that monotonically decreasing NO.sub.x production with soot-free operation at even higher equivalence ratios is possible. Achieving such an operation would require good fuel preparation, especially good fuel-air mixing.
The successful application of staged rich/lean combustion at all equivalence ratios is affected by the degree of fuel preparation, such as atomization and vaporization if the fuel is a liquid, and fuel-air mixing. Up to now, the only methods of preparing the fuel for staged rich/lean combustion have been fuel and air fluid dynamic processes. However, these processes have not been capable of producing the degree of fuel preparation required to achieve the soot-free, monotonically decreasing NO.sub.x production predicted by equilibrium thermochemical calculations.
Another problem encountered with rich/lean combustion is providing adequate cooling for the rich combustor. Conventional air film techniques cannot be used to cool the wall of the rich combustor because the cooling air would lower the equivalence ratio in the rich stage, reducing or eliminating the benefits of rich combustion.
Accordingly, what is needed in the art is a method and system for rich/lean combustion which provides improved fuel preparation and adequate rich combustor cooling.